KR20130129467A - Method for producing a poorly soluble calcium-arsenic compound - Google Patents

Abstract

The present invention relates to a process for the preparation of pentavalent calcium arsenate from an acidic solution, wherein the arsenic is at least partially in trivalent form. After neutralizing the acidic solution, it is subjected to an arsenic oxidation step and the poorly soluble calcium-arsenic compound is precipitated from the solution, where all arsenic is pentavalent.

Description

Method for producing poorly soluble calcium-arsenic compound {METHOD FOR PRODUCING A POORLY SOLUBLE CALCIUM-ARSENIC COMPOUND}

The present invention relates to a method of precipitating pentavalent calcium arsenate from an acidic solution, wherein the arsenic is at least partially in trivalent form. After neutralizing the acidic solution, it is subjected to an arsenic oxidation step and the poorly soluble calcium-arsenic compound is precipitated from the solution, where all arsenic is pentavalent.

Arsenic is naturally produced in many different forms. Sulphide minerals sometimes also contain arsenic in addition to the precious metals themselves, and therefore arsenic-containing mine water and other industrial wastewaters also sometimes arise in connection with the recovery of precious metals. Arsenic is also the most important impurity to be removed in connection with the recovery of nonferrous metals. The use of arsenic did not increase with respect to its recovery, so most of arsenic should be stored in waste form. Since arsenic and its compounds are toxic, they should be removed from the process after returning to the poorly soluble form as much as possible. The most sparingly soluble arsenic compounds in the neutral pH range are, for example, zinc, copper and lead arsenates, but binding arsenic to these precious metals has not been seriously considered due to the precious metal content that will remain in the waste. A widely used arsenic precipitation method is the precipitation of arsenic with iron as a very poorly soluble iron arsenate. In particular, the crystal form of iron arsenate, scorodite, FeAsO ₄ .2H ₂ O is less soluble than its other forms, amorphous iron arsenate. Another fairly stable compound on which arsenic precipitates is calcium arsenate.

Typically, arsenic is typically produced in solids and solutions as trivalent or pentavalent compounds. The trivalent form of arsenic is 60 times more toxic than the pentavalent form. In addition, it has been found that bad bodies precipitated in trivalent form, such as calcium arsenite, are not as stable as the corresponding pentavalent compound calcium arsenate, or are not normally approved for storage. Nevertheless, for example, more than 30% of the mine water may be in the form of arsenite, in which case trivalent arsenic must be oxidized five times before precipitation.

Arsenic removal from wastewater and mine water is described, for example, in US Pat. Nos. 5,114,592 and 5,378,366. U.S. Patent Publication 5,114,592 describes the precipitation of arsenic as calcium-magnesium arsenate by adding at least one calcium compound and at least one magnesium compound to an arsenic-containing waste solution in a pH range of 2 to 12, preferably 9 to 11. . The amount of arsenic in the solution is several thousand milligrams per liter. Prior to precipitation, trivalent arsenic in the acidic or alkaline range of pH values is oxidized in 5 rows with a suitable oxidizing agent such as calcium peroxide CaO ₂ , magnesium peroxide MgO ₂ or hydrogen peroxide H ₂ O ₂ . After precipitation of the calcium-magnesium arsenate and liquid-solid separation, the remaining arsenic can be further separated from the aqueous solution by adsorbing with activated carbon or by removing arsenic by ion exchange.

In the method disclosed in U.S. Patent Publication 5,378,366, it is essential that the treated arsenic-containing water is mainly groundwater or wastewater, with the amount of arsenic being approximately 2 mg / l (2000 ppm). The temperature of the aqueous solution is first raised to a range of 35 to 100 ° C. Subsequently, arsenic in the solution is oxidized 5 times using a strong oxidizing agent. After this, the calcium compound is treated with the solution to precipitate arsenic as calcium arsenate. Precipitation of calcium arsenate is carried out in a very alkaline pH range, values of about 11 to 13.

Object of the invention

The present invention relates to a method for removing arsenic from an acidic aqueous solution produced in connection with a metallurgical process, wherein the arsenic is at least partially in trivalent form in solution, the concentration of which is several times higher than those set forth in the prior art.

The present invention provides a method for preparing a pentavalent calcium-arsenic compound from an acidic feed solution containing trivalent arsenic, wherein the solution is neutralized with a magnesium compound, and then the solution is subjected to an oxidizing step, wherein arsenic is removed using a strong oxidizing agent. And oxidize in valent form and then precipitate arsenic as a poorly soluble calcium-arsenic compound from solution with the aid of calcium compounds.

According to one preferred embodiment of the invention, the magnesium compound used for neutralization of the feed solution is magnesium hydroxide, Mg (OH) ₂ .

According to a preferred embodiment of the invention, the calcium compound used for the precipitation of arsenic is calcium hydroxide, Ca (OH) ₂ or calcium oxide, CaO.

According to a preferred embodiment of the invention, the precipitated calcium-arsenic compound is one or more different forms of calcium arsenate.

According to a preferred embodiment of the invention, the strong oxidizing agent is at least one of: oxygen and / or sulfurous acid, ozone or hydrogen peroxide.

According to an embodiment of the invention, the gypsum is also removed from the solution together with the precipitated calcium-arsenic compound.

According to a preferred embodiment of the invention, after precipitation and separation of the calcium-arsenic compound, the magnesium in solution is precipitated as magnesium hydroxide Mg (OH) ₂ with the calcium compound.

According to an embodiment of the invention, one part of precipitated magnesium hydroxide is fed back to neutralization (1) of an acid feed solution comprising trivalent arsenic.

According to an embodiment of the present invention, a second portion of precipitated magnesium hydroxide is fed to an oxidation step (2) in which trivalent arsenic is oxidized five times.

According to an embodiment of the invention, the gypsum in solution is precipitated from the solution after the arsenic oxidation step to form a pure gypsum deposit.

1 shows a flowchart of an embodiment of a method according to the invention.

The object of the process according to the invention is to remove arsenic from the acidic aqueous solution produced in connection with metal production. Such aqueous solutions may also be formed in connection with gas scrubbing and may be, for example, impure solutions of sulfuric acid such as spent acid. The aqueous solution to be treated may contain thousands of grams of arsenic per liter, which should be removed to such an extent that the solution is recycled back to leaching, gas scrubbing or another process step. When an aqueous solution is used to leach metal from minerals comprising the metal, typically the aqueous solution comprises an acid and the pH may be approximately 0-1. Arsenic in the solution is at least in part because the third type (As ^{3 +)} to be oxidized into the (As ^{+ 5)} 5 before precipitation.

The method according to the invention is described herein by FIG. 1. The acidic feed solution should be neutralized in neutralization step 1 to a pH value at which no free acid is present in the solution and subjected to oxidation step 2 of trivalent arsenic. In principle, any neutralizing agent, such as CaCO ₃ , Ca (OH) ₂ , CaO, MgO, NaOH or KOH, can be used as the acid neutralizing agent. However, in developing the process according to the invention, when neutralization is carried out with the above-mentioned calcium compounds, it is found that some of the arsenic initially reacts with calcium in this step and tries to form calcium arsenite, an undesirable compound. It was. At the same time, the calcium-based neutralizer forms gypsum deposits with sulfuric acid in solution. In such cases, the final product is gypsum as well as waste deposits comprising both trivalent and pentavalent arsenic. In addition, it is difficult to control the precipitation in order to deposit the desired amount of trivalent or pentavalent arsenic precipitate. On the other hand, if, for example, potassium hydroxide or sodium hydroxide (KOH, NaOH) is used as a neutralizer, precipitation problems can be avoided, but the solution is recycled to remove excess sodium and potassium as it is collected in the process. This requires a separate bleed stream to do so, which in turn increases the overall cost of the process.

If neutralization of the acid in solution with magnesium compounds, for example magnesium hydroxide (Mg (OH) ₂ ) is carried out according to the invention, precipitation of trivalent or pentavalent arsenic is not yet produced in the neutralization step. Magnesium sulfate does not form a precipitate under these conditions, but remains in solution.

H ₂ SO ₄ + Mg (OH) ₂ → MgSO ₄ + 2 H ₂ O (1)

The neutralization solution is treated with oxidation step 2, wherein the five transverse oxidation of trivalent arsenic is carried out using known oxidants, for example by using oxygen and sulfurous acid, ozone or hydrogen peroxide. The pH range of oxidation is not so accurate when using the above mentioned strong oxidizers. According to the following equation, trivalent arsenic is oxidized 5 times:

^{_{3AsO 2 - + O 3 (g}} ) + 3H 2 O = 3H 2 AsO 4 - (2)

Since the pentavalent arsenic (acid) formed is a stronger acid than the trivalent arsenic (acid), the pH of the solution is lowered in the oxidation process, for example using a magnesium hydroxide-gypsum sediment recycled from a later step. Neutralizes:

^{_{3AsO 2 - + O 3 (g}} ) + 1.5Mg (OH) 2 = 3HAsO 4 2 - + 1.5Mg 2 + (3)

Gypsum in the precipitate, CaSO ₄ · 2H ₂ O, does not dissolve under these conditions, and therefore does not prevent oxidative neutralization. In this step, a slurry of a solution is formed which comprises precipitates which are pentavalent arsenic and mainly gypsum. Prior to precipitation of arsenic as a calcium-arsenic compound, gypsum deposits can be separated from the arsenic (V) solution by liquid-solid separation (not shown in detail in the figure). Gypsum deposits can be transferred, for example, to different dump sites, and in the following steps, pure calcium arsenate deposits can be deposited. If necessary, since the metal in the solution is in hydroxide form, the remaining arsenic and other metals can be cleaned from the precipitated gypsum deposit by first using an acid-containing solution. If the feed solution is a solution produced or formed in connection with metal production, the other metals are, for example, iron, copper, nickel and zinc. Another alternative presented in FIG. 1 is to terminate in the same waste site by omitting liquid-solid separation and precipitating calcium arsenate with gypsum deposits.

After the arsenic oxidation step, the calcium compound is fed into a solution, such as calcium hydroxide, Ca (OH) ₂ , ie slaked lime or calcium oxide, CaO, ie quicklime, to precipitate arsenic from the solution in precipitation step 3. For precipitation, the pH of the solution is adjusted in the range of 6 to 9, ie the range in which magnesium in the solution does not yet initiate precipitation as hydroxide but precipitates calcium-arsenic compounds. Precipitation is produced at the same temperature as the other solution treatments, ie generally in the range from 25 to 75 ° C. If arsenic precipitates out of solution with various forms of calcium arsenate, and the gypsum is not separated at an early stage, it is present in the deposit. The slurry is treated with solid-liquid separation 4 and the precipitated solid is separated from the solution.

Calcium-arsenic compounds are precipitated with calcium hydroxide as follows:

H ₃ AsO ₄ + 2Ca (OH) ₂ = Ca ₂ AsO ₄ OH + 3H ₂ O (4)

The exact form of the precipitated compound depends on the pH value of the precipitation step, although several compounds may be present in the dark complex, but different forms of calcium arsenate. The resulting calcium-arsenic compound is more stable than the compound formed in the higher pH range because precipitation must be carried out at a pH range of less than 9 to avoid co-precipitation of magnesium.

After arsenic removal, the solution still contains the dissolved magnesium sulfate produced upon neutralization, so that the solution in Mg precipitation step 5 using a calcium compound (calcium hydroxide or oxide) in the pH range of 9 to 11, preferably 9 to 10 Magnesium is precipitated from the mixture as magnesium hydroxide.

MgSO ₄ + Ca (OH) ₂ → Mg (OH) ₂ + CaSO ₄ (5)

In Mg precipitation, since the pH rises above 9, perhaps other metals contained in the solution are also precipitated. Only alkali metals, such as sodium or potassium, do not precipitate, so when using alkali-based neutralizers the alkali concentration in the solution increases due to recycling, and their removal from the process requires a separate treatment step as mentioned above.

The formed slurry is treated with solid-liquid separation 6, wherein the Mg hydroxide precipitate is separated from the solution. The first part of the precipitate is fed back to neutralization step 1 of the arsenic-comprising aqueous solution and the second part to arsenic oxidation step 2 again. In these steps, magnesium hydroxide acts as a neutralizing agent. Gypsum precipitated with Mg hydroxide does not dissolve under aqueous neutralization conditions and therefore does not lead to precipitation of trivalent arsenic. As mentioned above, the pentavalent arsenic formed by oxidation is usually arsenic acid (the formation lowers the pH value of the solution), and magnesium hydroxide also functions as a neutralizing agent in this step.

After liquid-solid separation, the purified aqueous solution of arsenic and magnesium may be recycled back to the process of treating the arsenic-containing solution by the arsenic oxidation and precipitation process without separate purification and removal steps.

Even if the chemical used in the process in the precipitation of the calcium-arsenic compound is calcium-based, since the neutralization of the acid feed solution is performed using the magnesium compound, the precipitation of pentavalent arsenic as the calcium-arsenic compound can be controlled. Alternatively, individual gypsum and calcium-arsenic deposits can be produced in the process, for example due to low disposal costs. The process is economical only because calcium compounds are used here as precipitation chemicals.

Claims (10)

A process for preparing a pentavalent calcium-arsenic compound from an acid feed solution containing trivalent arsenic, wherein the solution is neutralized with a magnesium compound (1) and then treated with an oxidation step (2), wherein arsenic is used with a strong oxidizing agent. A process for producing arsenic as a poorly soluble calcium-arsenic compound (3) from the solution with 5 transverse oxidation, followed by calcination. The method according to claim 1, wherein the magnesium compound used for neutralization is magnesium hydroxide Mg (OH) ₂ . The method according to claim 1 or 2, wherein the calcium compound used for arsenic precipitation is calcium hydroxide, Ca (OH) ₂ or calcium oxide, CaO. The method of claim 1, wherein the precipitated calcium-arsenic compound is one or more different forms of calcium arsenate. The method of claim 1, wherein the strong oxidizing agent is one or more of the following: Oxygen and / or sulfurous acid, ozone or hydrogen peroxide. The method according to any one of claims 1 to 5, wherein the gypsum is also removed from the solution together with the precipitated calcium-arsenic compound. The method according to any one of claims 1 to 6, wherein after precipitation and separation of the calcium-arsenic compound (4), magnesium in the solution is precipitated (5) as magnesium hydroxide Mg (OH) ₂ using a calcium compound. . 8. Process according to any of the preceding claims, wherein the first portion of precipitated magnesium hydroxide is fed back to neutralization (1) of an acid feed solution comprising trivalent arsenic. 9. The process according to any one of claims 1 to 8, wherein a second portion of precipitated magnesium hydroxide is fed to an oxidation step (2) in which trivalent arsenic is oxidized five times. 10. The method according to any one of claims 1 to 9, wherein the gypsum in solution is precipitated from the solution after the arsenic oxidation step (2) to form the pure gypsum deposit.

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